Taking a tougher line than the Trump administration, a spokesperson for the UK's Foreign Office said, if media reports surrounding the case were correct, the UK would treat the incident "very seriously."

Earlier on Tuesday, US President Donald Trump said he had not yet spoken to Saudi officials about the journalist's disappearance.

"I have not. But I will be at some point," he told reporters. "I know nothing right now. I know what everybody else knows - nothing."

In a separate development, Mr Khashoggi's Turkish fiancée Hatice Cengiz appealed to the US for help.

But after several of his friends were arrested, his column was cancelled by the al-Hayat newspaper and he was allegedly warned to stop tweeting, Mr Khashoggi left Saudi Arabia for the US.

What's been the reaction to the disappearance?

US Secretary of State Mike Pompeo called on Saudi Arabia to "support a thorough investigation" of his disappearance and "to be transparent about the results".

UN experts have demanded a "prompt independent and international investigation" into his disappearance.

Last week, Crown Prince Mohammed told Bloomberg News that his government was "very keen to know what happened to him", and that Mr Khashoggi had left "after a few minutes or one hour".

An unacceptable line?

Analysis by BBC diplomatic correspondent James Robbins

The UK has apparently told the Saudis they need to show Mr Khashoggi is alive, and the best way to do that would be for him to appear on television.

If that doesn't happen soon, it's clear that Britain and its allies are likely to conclude that Saudi Arabia has crossed an unacceptable line.

In the wake of the Salisbury attack, and the international punishment meted out to Russia, it would then be increasingly difficult for Western governments to avoid action against the Saudi kingdom, however close their military and economic ties.

Crown Prince Mohammed's brother and the Saudi ambassador to the US, Prince Khaled bin Salman al-Saud, has insisted all the reports about his disappearance or death "are completely false and baseless".

"Jamal has many friends in Saudi Arabia, and I am one of them," he said in a statement, saying the two had kept in touch while he was living abroad "despite differences".

Saudi Arabia says the allegations are baseless. It has allowed journalists into the consulate to show Mr Khashoggi is not there, reportedly even opening cupboards.

The BBC's Security Correspondent Frank Gardner says one possible scenario is that he was abducted inside the consulate, driven out in a diplomatic vehicle and "rendered" back to Saudi Arabia to either face retribution or be held incommunicado under indefinite house arrest.

Image copyrightAFPImage caption
The Saudi consulate said Khashoggi left after completing paperwork

BBC Newshour interviewed the journalist just three days before his visit to the consulate, and in an off-air conversation asked if he would ever return to his home country.

"I don't think I'll be able to go home," he told the BBC, saying that in Saudi Arabia "the people who are arrested are not even dissidents" and saying he wished he had a platform at home to write and speak freely at this time of "great transformation" in his country.